1. Field of the Invention
This invention relates to optical control of microwave circuits and systems and more particularly to an optical gain control circuit for controlling the gain of a gallium arsenide (GaAs) microwave monolithic integrated circuit (MMIC) distributed amplifier.
2. Description of the Prior Art
The utilization of light to control microwave devices and microwave subsystems is highly desirable because the light signals could be distributed and handled by optical fibers which offer advantages such as high speed, large bandwidth, good electrical isolation and elimination of grounding problems. Additionally, the optical fiber itself is small in size and light in weight and is immune to electromagnetic interference and electromagnetic pulses. The use of optical signals for controlling microwave systems also permits optical computing and optical signal processing techniques to be utilized. Because of the foregoing advantages, the development of effective and reliable optical control systems to control the microwave and millimeter wave devices used in radar systems and the like is very important. For example, the developement of these systems could lead to remote control of the transmit-receive modules which are used in phased array antenna systems or the optical control technique could be utilized to control the microwave amplifiers which are used in antenna beamforming applications. Since microwave devices and systems are fabricated utilizing monolithic integrated circuit technology wherever possible because of cost, size, weight and overall performance considerations, it would also be desirable to deveope light control systems for microwave devices which utilize this technology.
Although some attempts have been made to control the performance of microwave devices, such as Impatt diodes and field effect transistors FET's), for example, by direct optical illumination, the results have not been particularly successful. These attempts have utilized high speed, high frequency FETs and optical fibers to inject light into the active region of the FET between the gate and the source or the drain, so that the absorbed photons from the light generate electron-hole pairs which alter the performance of the FET. The difficulty with this work has been the poor coupling of the light into the active region of the FET and the resulting limited change in the performance of the FET. The geometry of a high speed, high frequency FET, namely, its relatively large gatewidth which is of the order of several hundred micrometers and very short gatelength which is of the order of a micrometer or less, is fundamentally incompatible with a typical cylindrical light spot emerging from the end of an optical fiber.